Ventricular fibrillation is the main mechanism of sudden cardiac death. Recent studies revealed that premature beats originate from the Purkinje fibers, a specialized intraventricular conducting system. The premature beats were found to originate from both the left and right ventricles. These premature beats are the dominant sources of iodiopathic ventricular fibrillation and accompanying close coupled ventricular ectopic beats. These sources can be eliminated by local energy delivery, for example, by ablation using radio frequency energy. It would be desirable to provide a device that could easily and efficiently map the Purkinje fibers to permit effective targeted radio frequency delivery to eliminate premature beats and local Purkinje potentials.